The Boat is Sinking
Water vapor is a naturally occurring greenhouse gas and it is increasing in our atmosphere. This is increasing global temperatures, but human actions are still to blame.
How is this possible? Think about sitting in a boat.
Imagine that you are sitting in a boat in the ocean when all of a sudden a shark takes a bite out of the bottom of your boat. Your boat begins to take on water and eventually sinks. What caused your boat to sink? Someone may say that the shark was the culprit. But someone else might argue that the water in the surrounding ocean was to blame. How could we possibly pin this on the water?
Think about it this way: The hole in the boat would be harmless if the ocean were empty. However, since there was water in the ocean, some water entered the boat through the hole and added weight to it. Once there was added weight, the boat sunk a bit and more water entered, causing the boat to sink more, and so on. This process continued until the boat was full of water and then completely submerged.
We can therefore conclude that a positive feedback occurred between the weight of the boat and the incoming water, ultimately causing the boat to sink. It was the shark, however, that started the process. The shark taking a bite out of your boat can be thought of as humans who are emitting greenhouse gases into our atmosphere.
Water vapor is a naturally occurring greenhouse gas in our atmosphere, but the amount of vapor in the air depends on temperature. Warmer air can hold more water vapor.
Another important greenhouse gas, carbon dioxide or CO2, also occurs naturally in our atmosphere. The amount of atmospheric CO2, however, has been increasing as humans continue to release carbon into the atmosphere through the use of fossil fuels. In fact, CO2 concentrations have increased by nearly 50% since pre-industrial times. This increase has resulted in higher global temperatures. Just as the boat took on more water as it began to sink, our atmosphere is holding more water vapor as temperatures rise.
Remember that water vapor itself is a greenhouse gas. This means that more of it in our atmosphere will further increase temperatures. As temperatures again rise, more water vapor is present, causing temperatures to rise, resulting in more water vapor... And the process continues.
We can therefore conclude that increasing atmospheric water vapor is warming our planet. However, by emitting other greenhouse gases, humans started the process, like a shark taking a bite out of a boat. Given the vastness of our oceans, there is plenty more water to spare.
This ongoing cycle is called a positive feedback and it approximately doubles the amount of warming that would occur from increases in CO2 alone. Such feedbacks are of particular concern for climate scientists since they have a tendency to amplify human actions.
Importantly, our shark analogy differs from climate change in following way. Instead of one small increase of atmospheric CO2 (one shark bite), we are adding more and more every year (several shark bites in a row causing a bigger and bigger hole).
For more information, check out a Skeptical Science article about how the water vapor greenhouse effect works.